一个ll Linear Algebra Resources
Example Questions
Example Question #21 :The Inverse
andare both two-by-two matrices.has an inverse.
True or false: Bothandhave inverses.
False
True
True
一个matrix is nonsingular - that is, it has an inverse - if and only if its determinant is nonzero. Also, the determinant of the product of two matrices is equal to the product of their individual determinants. Combining these ideas:
If eitheror, then it must hold that
.
Equivalently, if eitherorhas no inverse, thenhas no inverse. Contrapositively, ifhas an inverse, it must hold thateachofandhas an inverse.
Example Question #21 :The Inverse
andare both nonsingular two-by-two matrices.
True or false:must also be nonsingular.
False
True
False
We can prove that the sum of two nonsingular matrices need not be nonsingular by counterexample.
Let,.
一个matrix is nonsingular - that is, with an inverse - if and only if its determinant is nonzero. The determinant of a two-by-two matrix is equal to the product of its upper left to lower right entries minus that of its upper right to lower left entries, so:
Bothandare nonsingular.
Now add the matrices by adding them term by term.
,
the zero matrix, whose determinant is 0 and which is therefore not nonsingular.
Example Question #21 :The Inverse
is a singular four-by-four matrix. True or false:must also be a singular matrix.
True
False
True
一个matrix is singular - that is, it has no inverse - if and only if its determinant is equal to 0.is singular, so
.
The determinant of the scalar product ofand anmatrixis
;
setting,,:
Therefore,, having determinant 0, is also singular.
Example Question #21 :The Inverse
is a nonsingular matrix.
True or false: the inverse of the matrixis.
False
True
True
By definition,
and.
Multiply:
Similarly,
Therefore,is the inverse of.
Example Question #22 :The Inverse
True or False: If,are square and invertible matrices thenis also invertible.
True
False
True
To proveis invertible, we need to find another square matrixsuch that.
Sinceexist, take, then we have
,
and
.
Henceis invertible.
Example Question #22 :The Inverse
Suppose thatis an invertible matrix. Simplify.
To simplify
we used the identities:
so we get
Example Question #21 :The Inverse
Suppose thatare all invertible. What is the inverse of?
The inverse ofissince we can multiply it byto get:
Thereforeis the inverse of
Example Question #28 :The Inverse
Find.
does not have an inverse.
The inverse of a two-by-two matrix
is
Substituting the entries in the matrix for the variables:
Example Question #22 :The Inverse
Find.
To find the inverse of a matrix, set up an augmented matrix, as shown below:
Perform row operations on this matrix until it is in reduced row-echelon form.
The following operations are arguably the easiest:
The augmented matrix is in reduced row-echelon form. The inverse is therefore
.
Example Question #21 :The Inverse
.
Calculate.
is undefined.
is undefined.
The matrix is not a square matrix - it has two rows and three columns - so it does not have an inverse.
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